Looking to upgrade gfx card

As the title says I am looking to upgrade the Gfx card on my current build but i'm unsure whether it is worth just upgrading the gfx atm or will i need to to replace more?
Here's my current setup;
Computer - Advent Centurion CPD 1303

Ram - 4GB DDR2 (2 memory slots. Max 4GB)

CPU - AMD Phenom(tm) II X2 545 Processor 3.00Ghz

PSU - 405Watt, Make - CWT, Model - PUFP405P

GPU - ATI Radeon HD5550 512MB

Mobo - Foxconn A7VMX-K

Operating System- Windows 7 Home Premium 64 bit

Any recomendations on a good upgrade for this? Budget is around £150. Though could stretch another hundred if needed to replace more
Not looking to play Crysis 3 at Ultra, just run Dead Rising 2, WoW and a few other semi intensive games at high or greater settings.

I'd look at the APU market: for example, pcpartpicker.com/uk/p/vP0L that I cooked up at PCPartpicker replaces most of the things, and should be able to fit in your case. It's a bit higher than your budget at £212, but it does include a new PSU - for an extra £40, you'd be able to get a Modular PSU. IF you know someone who can put it all together, then you should be fine.

A word of warning, though - your current CPU cooler may not be able to fit the newer FM2 boards.

Alternatively, novatech.co.uk/products/barebones/bb-58004g.html from Novatech should cover most of it, but as it's £215, that might also be a little outside your budget.